You can reference Kent Mechanical Engineering Handbook "Power". My 12th edition(9th printing,1/1967) pg3-20 Turbulent Flow of Liquids Inside Helical Coil" may answer your question. The formula for the coefficient of convective heat transfer(h*) for straight pipe is too complicated, however, if you can get it from Kent or other references; then according to Kent the "h" value for helicoil can be h= (1+(3.50(d/d*))h* whereby d/d* is the ratio of pipe diameter to the helix diameter. There will be some caution presented in Kent such as reduction of h due to corrosion and dirt; the effect of pipe length may be negligible when length exceed 20 diameters; for shorter length h may be appreciably higher. There is no mention as to the effect of the pitch on the h value.